The need for having available in the industrial practice catalysts supported on magnesium dihalides in active form, endowed with a high activity and capable of providing polymers with a controlled morphology is more and more felt.
Examples of catalysts having a controlled morphology are described in U.S. Pat. Nos. 3,953,414 and 4,399,054. In the latter patent, the components are prepared from spherically shaped adducts of MgCl.sub.2 with approximately 3 mols of alcohol. Prior to the reaction with TiCl.sub.4, the alcohol content is decreased down to 2.5-2 mols: in this manner, components are obtained which exhibit a porosity, as measured with nitrogen, of from 0.3 to 0.4 g/cm.sup.3 and an average pore radius comprised between 15 and 20 .ANG..
Catalysts prepared from TICl.sub.4 and MgCl.sub.2 in granular form, by spray drying an alcohol solution of magnesium chloride and subsequently supporting the titanium compound, are described in patents EP-B-65700 and EP-B-243327. However, the polymer obtained using these catalysts does not exhibit morphological characteristics of interest. In particular, the bulk density is not sufficiently high. Furthermore, the activity of the catalyst is rather low.
A method for increasing the activity of these catalysts is described in patent EP-A-281524. These catalysts are prepared by supporting titanium alkoxides onto an MgCl.sub.2 -ethanol adduct, containing from 18 to 25% by weight of ethanol, spherulized by spray-drying of an ethanol solution thereof and subsequent chemical treatment with Et.sub.2 AlCl or Et.sub.3 Al.sub.2 Cl.sub.3. The conditions for preparing the support are critical and have an influence on the morphological stability of the resulting polymer. For example, polymers in the form of heterogeneous powder are obtained when supports are used having an alcohol content which is not comprised within the range of 18-25%, or compounds are used which are different from Et.sub.2 AlCl or Et.sub.3 Al.sub.2 Cl.sub.3. Furthermore, in order to achieve high yields the Ti content in the solid component is always greater than 8% by weight.
From patent application EP-A-395083, catalysts suitable for producing spherically shaped polymers endowed with satisfactory morphological properties are known, obtained from MgCl.sub.2 -alcohol adducts, in which the adduct, generally containing 3 mols of alcohol per mol of MgCl.sub.2, is dealcoholated by thermal treatment down to alcohol levels generally comprised between 0.2 and 2 mols, and is thereafter caused to react with an excess of titanium tetrachloride optionally containing an electron donor compound dissolved therein.
The solid components of these catalysts are characterized by a high specific surface area and microporosity (more than 50% of the pores has an average radius smaller than 800 .ANG.).
When used for the polymerization of ethylene to produce LLDPE, these catalysts lead to a comonomer distribution which is not sufficiently uniform.